Biology The movement of a substance, such as a liquid or solute, across a cell membrane by means of diffusion or osmosis.

Chemistry The process by which one substance, such as a solid or liquid, takes up another substance, such as a liquid or gas, through minute pores or spaces between its molecules. A paper towel takes up water, and water takes up carbon dioxide, by absorption. Compare adsorption.

Physics The taking up and storing of energy, such as radiation, light, or sound, without it being reflected or transmitted. During absorption, the energy may change from one form into another. When radiation strikes the electrons in an atom, the electrons move to a higher orbit or state of excitement by absorption of the radiation's energy.